check if movieclip track has segment
This seems to be a good check:
track = bpy.data.movieclips['some_clip'].tracking.tracks['some_track']
len(track.markers) &lt;= 3

As I understand it, each track has one marker per tracked frame and for some reason, some tracks with no segment can still have up to 3 markers.
There might be a better solution but so far I haven't had an issue with that.